The 34-year-old Williams told the Baltimore Ravens on Tuesday he won't be back to fulfill the second year of a contract he signed in August. Playing as a backup to Ray Rice this year, Williams ran for 444 yards and scored two touchdowns.

He also became the 26th player in NFL history to reach 10,000 yards rushing, reaching the plateau in the season finale at Cincinnati.

"The NFL has been an amazing page in this chapter of my life," Williams said. "I pray that all successive adventures offer me the same potential for growth, success and most importantly, fun. ... As for what's next, I am excited about all the opportunities ahead — continuing my education, running The Ricky Williams Foundation and whatever other opportunities present themselves."

Williams retired previously before the 2004 season when facing a four-game suspension for violating the league's drug policy. He returned in 2005, then left to spend the 2006 season with Toronto of the Canadian Football League.

After an outstanding college career at Texas, where he won the Heisman Trophy, Williams broke into the NFL in 1999 with the New Orleans Saints. Mike Ditka, then coach of the Saints, traded all the team's draft and first- and third-round picks in 2000 to pick Williams fifth overall.

Williams spent only three years with New Orleans, but during over 11 years in the league he had five 1,000-yard rushing seasons and finished with 10,009 yards on the ground.

Williams led the NFL in 2002 with 1,853 yards rushing for the Miami Dolphins and received his lone Pro Bowl invitation that season. He backed that up with 1,372 yards in 2003, giving him what remains the two most productive rushing seasons in Dolphins history.

Although Williams' last start was in 2009, he enjoyed coming off the bench for the Ravens.

"It's been interesting," he said in November. "It's been an adjustment for me, but I love the organization and I love my teammates so I'm having a good time. I'm enjoying myself. Anytime you play a team sport, the success of the team really makes everything better. It's nice."

Baltimore went 12-4 this season and lost to New England in the AFC championship game.

During that November interview, Williams was asked whether playing as a backup could possibly extend his career.

"At this point, my focus is just to finish my career strong," he said. "I'm not even thinking two or three years down the line. But one positive about not carrying the ball is my body does feel good."

Williams scored 73 touchdowns over his career, all but eight of them on the ground. His last score came on Dec. 4 against Cleveland.

"I have to thank coach (John) Harbaugh and the Ravens organization for the opportunity they gave me this year," Williams said. "I had so much fun and really appreciated the chance to finish on such a great note."

Williams fit in well with the Ravens and made a lasting impression on Rice, who led the NFL this season with 2,068 yards from scrimmage.

"I was a big fan of Ricky before we were teammates, but being around him this year is the best thing that happened to me in my NFL career," Rice said. "As a young player, you need to be around a guy who knows what he is doing, and Ricky was tremendous to learn from. The way he took care of his body and the way he prepared, he always showed that he is a true professional. This past season with him is a year I will never forget.

"I had the best year with him beside me, and that was no accident. I believe that Ricky Williams is a Hall of Famer. All that he has done in his career, he deserves that."

Harbaugh said during his time in Baltimore, Williams "made a valuable and lasting contribution. I especially enjoyed getting to know him as a person, and I have the utmost respect for him.